Summary/Abstract In the US, diabetic retinopathy is the leading cause of blindness in working age adults and remains a public health problem throughout the world. The earliest manifestations of this eye disease are believed to originate in capillary dysfunction resulting in both over- and under perfusion of regional capillaries. And while these changes in microvascular structure have been identified as hallmarks of the disease, the earliest functional changes in this microscopic network remain unclear. Is microvascular flow impaired early before capillary structural changes, or does the formation of aberrant vessel patterns, as a consequence, profoundly change retinal capillary flow? Conventional retina cameras generally lack the necessary resolution to study capillary-level blood flow because the eye's optics blur the microscopic capillaries at the back of the eye. In this study, we develop and deploy a new retinal camera that turns the eye into a high-power microscope to study single cell blood flow the back of the living eye. Combined with the optical improvements of this adaptive optics camera which corrects for image blur, we have coupled two other innovations to image the movement of individual blood cells as they move through the tiniest of capillaries only 1/10th the thickness of a human hair. First, blood cells are not only microscopic, but they also move at fast rates of speed. To image these blood cells free of motion blur, the use of a high-speed camera is required. In this research project, we combine the blur-correcting optics with an exceptionally fast camera that can capture over 30,000 snapshots per second. This camera is focused at single capillaries and can image the blood cells as they flow by -one by one. This advancement allows us to measure blood cell speed and provide exact counts of the number of passing blood cells, two innovative measures of blood flow at the capillary level. A second innovation uses special light-scattering properties of blood cells to provide highly detailed images of blood cell boundaries against the vessel wall and surrounding tissue. The resultant images provide not only high resolution images of blood cells, but can also provide unprecedented measures of blood cell type and their deformation within microvessels of the eye. By tracking the progressive changes in capillary flow and microvascular structure over the course of diabetes from weeks- to-years, we seek to better understand the earliest events leading to vascular disease of the eye. In this study, we examine the impact of high blood sugar levels on a mouse model of human diabetes. Changes in single- cell blood flow will be non-invasively imaged over time to determine the impact of diabetes on the smallest vessels of the eye.